CEV Design Q&A Options

[nprev]

Starting this thread in hopes that we'll all have some detailed insight into the next generation of MSF engineering.

Noble goal, eh? Truth of the matter is that I was just looking at one of the innumerable 'CEV enroute to the moon' artist conceptions, and I realized that the solar arrays as depicted meant that the vehicle almost certainly has to remain in a fixed attitude with respect to the Sun for them to operate at maximum efficiency. IIRC, the Apollos had to spin a bit in order to maintain thermal equilibrium & avoid localized heating and/or freezing.

Am I missing something here in terms of trade-offs? Seems unwieldy at best to have the arrays mounted to a sort of slip-ring assembly (with mast articulation) to maintain solar lock while the main body rotates. Likewise, seems as if an extensive--and in terms of power & volatile requirements/risk, expensive--ECS would be needed to avoid the temperature differential problem if the S/C is intended to maintain a continuous power-positive attitude during transit.

Sure that there's a simple answer I've missed; would very much like to hear it! (Let me guess: LOTS of heat pipes?)

[dvandorn]

Y'all have painted the Apollo thermal problem with too broad a brush, I'm afraid. The issue wasn't with CSM systems -- the CSM was fitted with radiators that served to keep its internal temperatures within acceptable ranges, even in heat soak / cold soak conditions. The fact that CSMs were able to fly in Earth orbit without any problems without resorting to passive thermal control proves this, as does the fact that CSMs were used as crew ferry vehicles during Skylab, during which time they spent days at a time with roughly the same solar orientation (they had to, Skylab spent a large amount of time in solar inertial attitude so that the ATM could make long-term solar observations).

The thermal issue with Apollo was entirely and solely with the ablative used on the lunar CMs. A different ablative was used on CMs designed for return from the Moon than was used on Skylab CMs, for example. It had better characteristics under the higher heat loads encountered on a lunar entry trajectory than anything else out there. But it had a very nasty tendency to deteriorate when it got too cold.

The lunar CSMs were, during trans-lunar and trans-earth flight, faced with a much more complete blackbody environment on their shadowed sides than is found in orbit around a reflective body such as Earth, or even the Moon. The CSM systems could maintain temps in the CM and SM, no problem, even in that environment and even in cold-soak conditions. But the heat shield needed to be rotated in order to keep it from getting too cold and deteriorating (shredding is a better word).

That's the reason why the extra-long entry blackout was more worrisome than anything else on Apollo 13 -- that spacecraft had used a very badly controlled PTC on its way back to Earth, and by the end the CM and LM were mostly in shadow, with the sun shining mostly down the throat of the SPS engine bell. That heat shield got colder than any other during Apollo, and the great fear was that it would have deteriorated enough to be ineffective during peak heating. Those who knew the risk at the time took every second that blackout lasted longer than normal as additional confirmation that the crew had been incinerated. It was a very emotional moment for those people when contact was finally regained.

-the other Doug

[Jim from NSF.com]

I disagree. There wasn't a difference in the heatshields and the skylab CSM's did have mods for the LEO environment. The PTC was also for the systems in the SM. There was white paint on the CM and heaters were added

[dvandorn]

I disagree. There wasn't a difference in the heatshields and the skylab CSM's did have mods for the LEO environment. The PTC was also for the systems in the SM. There was white paint on the CM and heaters were added

There were some *minor* modifications made to the SMs of Skylab CSMs to add heaters for long periods in solar inertial orientation. But in this case, about the heatshield materials, you're wrong, Jim. The heatshield material used on Skylab CSMs was indeed different than that used on lunar flights. And the *primary* reason for PTC on lunar flights was heatshield thermal control. Look it up. (The ethylene glycol active thermal control system in the CSM was designed to maintain thermal control inside the vehicle throughout translunar and transearth cost, with the understanding that attitudes would be shifted *on occasion* to avoid hot spots. PTC was *only* introduced to address the heatshield thermal regime issue.)

Actually, the largest modifications made to Skylab CSMs had nothing to do with the thermal regime. They had to do with the seals used on the hypergolic fuel systems. Mainline Apollo CSMs (and LMs, for that matter) used a variety of plastic seal materials on the hypergolic fuel systems that fed the various engines. The useful lifetime of an Apollo CSM was measured from the first moment it was fueled. At that point, the seals in the fuel lines began to deteriorate. You could not fly an Apollo CSM or LM more than two months after it was first fueled without tearing it apart and replacing all of the seals. For Skylab, the plastic materials (nylon mostly, IIRC) were replaced by newer materials (metals, in some cases) which resisted the corrosive effects of the hypergolics better, and for longer periods. Unfortunately, they didn't work as seals *quite* as well as the original materials -- the fuel leakage in the Skylab 3 RCS fuel system occurred around one of these new, longer-lasting seals.

But, once again, if you delve deeply into the Skylab historical documents, you'll find that the heatshield materials used were modified to avoid the cold-soak deterioration issue that lunar Apollo heatshields were prone to. The Skylab chronology mentions the subject at least a couple of times.

-the other Doug